A number of enzyme catalyzed processes have been identified which modify the DNA molecule and its associated chromatin. These epigenetic processes modulate gene expression. The association of epigenetic dysfunction with human disease has grown out of detailed molecular and chemical biology at the cellular and sub-cellular level. In some cases, these associations have led to new therapeutics agents, which can modulate epigenetic processes and potentially rescue the epigenetic status in diseased tissue. Despite the increasing link between epigenetic status at a molecular level and human disease and treatment, there are a surprisingly limited number of tools that allow researchers to directly probe epigenetic processes in vivo. New technologies for human molecular imaging that can report on enzymes which catalyze epigenetic transformations will revolutionize our ability to translate basic research to human therapy. To address this critical need, we aim to develop radiotracers for positron emission tomography (PET) that can provide molecular-level epigenetic information. While we will ultimately develop a series of radiotracers for a number of epigenetic targets, here we propose studies that will lead to an in vivo imaging agent relevant across many human diseases including, among others, cancer, central nervous system disorders, heart disease, and inflammation. Specifically, we will systematically develop and optimize a PET radiotracer for imaging class-I histone deacetylases (HDACs). We will accomplish this goal by: 1) developing and applying a distinct iterative refinement model to identifying class-specific HDAC inhibitors that, by design, contain a functional group suitable for PET radioisotope incorporation and which meet certain physiochemical criteria; 2) Labeling appropriate precursor compounds and evaluating their in vivo imaging potential in detail in rodents; and 3) Optimizing top radiotracer candidates, performing non-human primate imaging, and assessing their potential for translation to humans. A key feature of this proposal is a research strategy that can be easily adapted to address other classes of HDAC agents and other epigenetic targets. The outcome of this research will be a new technology for imaging epigenetic processes in vivo that can be used in both preclinical research and human studies.